If you post animals at the end of the month, you can re-post them at the beginning of the next month, once the old add is wiped.
A lot of people might post at the beginning of the month, so you could wait till it slows, like post on the 7th day of each month. Its like a chess game.
There will only be a few pages of classifieds come the end of the month, so people will be able to see all the current classifieds.
The daily or weekly thing would be quite of hard to deal with for the mods, but in reality it would be ideal.
The monthly clearance thing was something I pushed for while I was a mod, and I'm glad to see it becoming implemented now.
Implementation for the nation!